A tundish is a large intermediate holding vessel for molten metal used in continuous casting processes, such as the continuous casting of steel. The tundish is, in effect, an intermediate process reservoir which receives a large quantity of molten metal from conveying ladles exiting a furnace in which actual melting of the ore occurs, and which then transfers the molten metal to a casting mold. A system of inlet and outlet nozzles controls the flow of molten metal into and out of the tundish.
The tundish itself is generally a steel vessel which is lined with several layers of a refractory composition. A permanent lining, generally of refractory brick, serves as an inner lining to protect the vessel. The permanent lining, in turn, is coated with a wearable and disposable lining, generally of a refractory composition which has been applied to the permanent lining by gunning, spraying or trowelling. The disposable lining is in direct contact with the molten metal in the tundish and protects the permanent lining from exposure to the molten metal.
During the casting process, some molten metal remains in the tundish and solidifies on the lining. The disposable lining itself can become cracked, or spalled and/or some of it may break away leaving the permanent lining exposed to molten metal. This type of damage to the disposable lining is due to expansion of the refractory material, caused by uneven thermal forces on it as it comes in contact with the molten metal, and by abrasive contact with the molten metal.
Periodically, the supply of molten metal in the tundish is not refilled from the ladles and the reservoir of molten metal is allowed to deplete by filling molds until the tundish is empty, so that the tundish can be removed from service for maintenance. Solidified metal adhering to the disposable lining, known as "skull", is first removed by scraping and chisselling it away. This sometimes causes further damage to the surface of the disposable lining over and above any damage caused by the molten metal during operation. The disposable lining is then repaired by gunning, spraying or trowelling additional refractory composition onto the damaged portions, before the tundish is returned into service Occasionally, damage to the disposable lining may be severe, or the time may have come in a routine cycle of operation after several interim repairs that the entire disposable lining is removed down to the permanent lining and the tundish is completely re-lined with disposable lining refractory composition by gunning, spraying or trowelling it on.
Ideally, the disposable lining material is a refractory which is lightweight, but strong, has good rapid heating properties during curing after application and is resistant to cracking, spalling and thermal and structural stresses during operation of the tundish. The material must be capable of expansion without a significant reduction in strength. The material should also be capable of complete removal from the tundish when complete relining is desired.
Refractory materials having a high magnesia (MgO) content have traditionally been used as the disposable lining material.
Magnesia-based coating refractories, alone, however, have a high thermal expansion. This can lead to a problem known as "bulging", in which the disposable lining separates away from the underlying permanent lining as the disposable lining expands during heating. This problem has previously been overcome by the inclusion of a plasticizer and a binder in the refractory composition. The use of a plasticizer has been observed to increase the refractory's ability to adhere to a surface to which it is applied, while a binder increases the composition's lateral strength characteristics and ability to withstand forces acting perpendicularly against it.
Refractory compositions having too high a moisture content are susceptible to explosive spalling during curing and drying after application. This is caused during the heat-up of recently applied refractory composition to dry and cure it, by the vaporization of the retained moisture in the refractory accompanied by a buildup of steam pressure in the layers, followed by the sudden explosive fracturing or chipping away of some of the refractory layer as the pressure is relieved.
In order to overcome the problem of explosive spalling, it has heretofore been known to mix certain ingredients with the refractory aggregate base to impart to the refractory composition a porous state over and above the naturally occurring porosity of the refractory aggregate material, thereby allowing moisture vapor to vent, and preventing the buildup of pressure in the refractory layer which causes the explosive spalling. Materials added to fulfill this function have previously included mineral fibers such as glass wool and asbestos, and various hollow-core aggregates.